Introduction
How does the presence of ionic material affect a CMC?
Amphiphilic surfactants have both polar and non-polar parts in their molecules. Whenever their concentration surpasses the crtitcal micelle concentration (CMC), surfactants molecules organize themselves in a special structures and form micelles. Micelles in aqueous environment are organized in the way that charged hydrophilic ‘head’ groups of surfactant molecules closer to the surface of a micelle and exposed to water. Whereas, hydrophobic ‘tails’ are directed inside a micelle to avoid contact with water molecules. There is electrostatic repulsion between charged head groups of the surfactant molecules. Therefore, the molecules tend to form a structure which allows to have maximum distance between charged ‘heads’ but keeping non-polar ‘tails’ inside the agglomerate. It is possible when the structure has a spherical shape. Also, the numbers of surfactant molecules and hence the size of a micelle are limited by the fact that the distance between charged groups should be enough to keep the structure stable.
Addition of ionic materials to the micellar solution makes the repulsion between polar groups weaker, thus, allowing micelles to grow easily. This decreases CMC values.
In what purity grades is SDS available?
SDS is usually available in molecular biology/ electrophoresis purity grade. It contains more than 99% of SDS. This grade can be used as a protein denaturant for molecular weight determination in gel electrophoresis.
What contaminants are likely on clean-looking glassware?
Glassware can be contaminated by detergent residues or grease from contact with fingers. This could be checked by wetting the surface of glassware by distilled water. The presence of contaminants will not allow to wet the surface uniformly and water will form separate droplets. In this case the glassware needs to be cleaned thoroughly and rinsed. Also, if glassware is rinsed only with tap water, the salts in the water might be crystallized on the surface and interfere with the experiment results. Therefore, it is very important to rinse glassware with distilled water after cleaning.
What are the main differences between micellar solutions and emulsions?
Micellar solutions are lyophilic dispersed systems that are characterized by a strong interaction between dispersed phase and continuous dispersed medium. They are formed spontaneously without any external factors and thermodynamically stable. Whereas, emulsions are lyophobic systems where the phase and the medium do not interact with each other. Emulsions are not formed spontaneously and mechanical agitation is needed. They are not thermodynamically stable. The dispersed particles may aggregate together after some time and separate from the continuous medium.
Which other techniques are used for the determination of CMC of surfactants?
Techniques of determination of the CMC are based on observing an abrupt change of physical and chemical properties of surfactants depending on concentration. One of these properties is surface tension which decreases as concentration increases and after reaching the CMC becomes constant. This method can be used for both ionic and non-ionic surfactants. However, if the solution has some impurities, it will be difficult to determine the CMC. Similar techniques measures osmosis pressure, index of reflection, turbidity, fluorescence etc.
Solubilization of dyes and hydrocarbons in micelles allows to determine the CMC of ionic and nonionic surfactants in aqueous and non-aqueous solutions. When the CMC is reached, solubilization of dyes and hydrocarbons abruptly increases. Solubilization is measured by methods based on light scattering or spectrophotometry. Measuring of scattered light intensity makes possible not only determine the CMC but also find molecular weight of a micelle.
Materials and Methods
The electrical conductance of SDS solutions was measured by electrodes. Stock solutions of SDS of 1.5 x 10-2 mol/l was provided. The temperature of the solution was kept constant at 25°C using a thermostat. 12 dilutions of the stock solution was made and the conductance of each solutions was measured.
Solutions for UV absorbance measurement were prepared by mixing a SDS stock solution of 0.04 mol/l and methylene blue of 2 x 10 -5 mol/l. Furhter, the series of dilutions was made using twice diluted methylene blue solution. Each final solutions were filtrated separately. UV absorbance of each final solutions were measured at 660 nm.
Results
The CMC determination by electrical conductance measurement
As can be seen from the results of electrical conductance measurement, the conductance increases as concentration grows (see table 1, figure 1, and figure 2). Figures 1 and 2 shows that sharp change occurs at the point where concentration reaches 0.80x10-2 mol/l. Therefore, the CCM of SDS should be close to this value.
Results of electrical conductance measurement of SDS solutions.
Figure 1
Electrical conductance versus SDS concentration
Figure 2
Electrical conductance/SDS concentration versus SDS concentration
The CMC determination by UV spectroscopy
The results showed that absorbance slightly increases as concentration grows (see table 2, figure 3 and 4). The abrupt change of absorbance occurs at the point where SDS concentration is 0.01 mol/l. Thus, the CCM value determined by UV spectroscopy is close to 0.01 mol/l.
Results of UV spectroscopy measurement of SDS solutions
Figure 3
Absorbance versus SDS concentration
Figure 4
Absorbance versus the log SDS concentration
The combined mean value for the CMC of SDS is 0.009 mol/l with a range of 0.002.
Discussion
The results of two methods for the CMC value should be the same. However, electrical conductance and UV spectroscopy methods showed different results for the CMC, 0.0080 and 0.01 mol/l respectively. According to Shan and Khan, the CMC value for SDS reported in the literature and proved by the most sensitive to the micellar environment method of fluorescence spectroscopy is 8.3x10-2 mol/l (189). The reference value is still in the range of the results. However, the CMC value obtained by the technique based on electrical conductance is closer to the reference value than the result of UV spectroscopy method. The reason or that might be human factor, like using of improperly cleaned glassware, inaccurately made dilutions etc. Also, one needs to take into consideration that the reference value is in the range of the obtained results and the fact that very low concentrations were used during experiment. It is worth suggesting that in order to get the results in close agreement with the reverence values, the gaps between concentrations in the dilution series should be less than in the procedure of the given experiment. This could be achieved by providing the stock solution with the concentration value with more significant figures. For example, for UV spectroscopy it should be 0.0450 mol/l instead of 0.04 mol/l.
The result from electrical conductance method show that conductance is an almost linear function of SDS concentration (see figure 1 and 2). Before micellization process starts, the addition of a water soluble surfactant increases the number of charged particles which contribute to conductivity. Once the CMC is reached, further addition of the surfactant gives rise to a number of micelles. However, number of monomers in the solution remains the same. Being larger in size, a micelle moves more slowly in the solution and do not contribute efficiently as a charge carrier (Hubbard 1044). This can be seen as a sharp decrease of conductance at the CMC point in figure 1.
The sharp change in absorbance at the CMC point during UV spectroscopy method (see figure 3 and 4) can be explained by the fact that methylene blue solubility sharply increases at the CMC point. Furthermore, Carrol and others state that “at SDS concentrations significantly below the CMC methylene blue forms noncovalent dimers and aggregates with SDS” (780). Once the CMC is reached methylene blue form complexes with the micelles of SDS that apparently has high absorbance of UV light. As can be seen from figure 3 and 4, there is a slight rise in absorbance before the CMC. Whereas, after a sharp rise of absorbance at the CMC, the measured values do not show any particular tendency to increase or decrease. If the measured range is wider after the CMC, it might be possible to observe some tendency. Apparently, the experiment needs to be repeated to ensure that there was no human factor that led to inaccurate results.
Conclusion
The determination of the CMC is based on observing sharp changes of physical and chemical properties of a surfactant solution as concentration increases. The obtained combined mean value is 9x10-3 mol/l with the range of 0.002 which is close to the reference value of 8.3x10-3 mol/l
Works Cited
Hubbard, Arthur. Encylopedia of Surface and Colloid Science. London: Taylor & Francis, 2002. Print.
Khan, Asad and Shah, Syed. “Determination of Critical Micelle Noncentration of Sodium Dodecyl Sulfate and the Effect of Low Concentration of Pyrene on its Cmc Using ORIGIN Software”. Journal of Chemical Society Feb. 2008: 186-191. Web. 2 Apr. 2014.
Carroll, Mary K. et al. “Interactions between Methylene Blue and Sodium Dodecyl Sulfate in Aqueous Solution Studied by Molecular Spectroscopy”. Journal of Society of Applied Spectroscopy July 1999: 780-784. Web. 2 Apr. 2014
